LAB & MLF Flashcards
What is the primary role of Lactic Acid Bacteria (LAB) in wine production?
LAB influence acidity, aroma, and overall quality of wine
LAB are involved in carbohydrate and organic acid degradation and can produce various by-products.
List the key genera of LAB that thrive in wine conditions.
- Lactobacillus
- Oenococcus
- Pediococcus
These genera are capable of surviving in the specific conditions of wine.
What are the two main types of fermentation carried out by LAB?
- Homofermentation
- Heterofermentation
Each type has distinct metabolic pathways and by-products.
Define Homofermentation.
A metabolic process where LAB primarily converts hexoses (glucose) into lactic acid via glycolysis
It generates 2 molecules of lactate and 2 molecules of ATP per hexose molecule.
What is produced during Heterofermentation?
- 1 lactate molecule
- 1 CO2 molecule
- Either 1 ethanol or 1 acetic acid molecule
- 1-2 ATP molecules
The production ratio of acetate and ethanol depends on environmental conditions.
What is the function of the Pentose Phosphate Pathway in LAB?
It influences the metabolism of sugars and by-products during fermentation
It is particularly relevant in the context of heterofermentation.
What is Malolactic Fermentation (MLF)?
The process by which LAB degrade malic acid into lactic acid and CO2
This is catalyzed by the malolactic enzyme using NAD+ and Mn2+ as co-factors.
What are the by-products of citric acid metabolism by LAB?
- Acetic acid
- Lactic acid
- Acetoin
- Diacetyl
- Butane-2,3-diol
The production of these metabolites varies by bacteria strain and environment.
True or False: Diacetyl at low concentrations imparts a buttery flavor to wine.
False
Diacetyl at low concentrations imparts yeasty, nutty, toasty aromas.
Fill in the blank: LAB can degrade _______ to produce either lactic acid and acetic acid or succinic acid and two acetic acid molecules.
[tartaric acid]
What undesirable compound can be formed from glycerol degradation by certain LAB?
Acrolein
Acrolein can create bitter-tasting complexes with phenolic compounds.
What harmful compounds can LAB produce from amino acids?
- Ethyl carbamate precursors
- Biogenic amines (e.g., histamine)
High levels of histamine can arise from a lack of carbohydrates and high amino acid levels.
What is the impact of environmental factors on LAB metabolism?
They significantly influence LAB activity and the formation of various by-products in wine
Factors include temperature, pH, and the presence of other substances.
What practices can help control LAB activity in winemaking?
- SO2 addition
- Maintenance of good winery hygiene
These measures are essential for producing stable and desirable wines.
What is malolactic fermentation (MLF)?
MLF is the conversion of L-malic acid to L-lactic acid by bacteria
This process is significant in winemaking as it alters the wine’s acidity and organoleptic properties.
What are the key impacts of MLF on wine?
MLF affects the pH, total acidity (TA), microbiological stability, and organoleptic properties of the wine
These changes can enhance the wine’s flavor and stability.
Which bacteria are primarily responsible for MLF?
Primarily Oenococcus oeni, but also Lactobacillus and Pediococcus
Oenococcus oeni is the most important due to its adaptation to wine conditions.
What is the difference between malic acid and lactic acid?
Malic acid is diprotic with two pKa values (3.56 and 5.1), while lactic acid is monoprotic with one pKa value (3.86)
This difference affects the perceived acidity and pH of the wine.
What are the effects of MLF on pH and total acidity (TA)?
MLF leads to an increase in pH (up to 0.3) and a decrease in TA (1-3 g/l)
The extent of these changes depends on initial malic acid levels and the wine’s buffering capacity.
Why does MLF occur?
The metabolism of malic acid improves wine conditions by raising the pH and allowing ATP energy formation in the cell membrane
This benefits the bacteria involved in the fermentation.
What are the general characteristics of Lactic Acid Bacteria (LAB)?
LAB are unicellular, smaller than yeast, prokaryotic, and reproduce by binary fission
They are also Gram-positive.
What is the difference between homofermentative and heterofermentative LAB?
Homofermentative LAB produce primarily lactic acid from glucose, while heterofermentative LAB produce lactic acid, CO2, acetic acid, ethanol, and other products
The transformation of malic acid to lactic acid is a side reaction in heterofermentative LAB.
What is the significance of Oenococcus oeni in wine?
Oenococcus oeni is the most important LAB in wine due to its better adaptation to wine conditions and cleaner MLF
It produces fewer negative side reactions compared to other LAB.
What phases of growth do LAB exhibit?
LAB exhibit lag, growth/exponential, stationary, and decline phases
The lag phase is longer for bacteria than yeast.
